The Sudden Surges That Forge Evolutionary Trees

Summary

Over the last half-billion years, squid, octopuses and their kin have evolved much like a fireworks display, with long, anticipatory pauses interspersed with intense, explosive changes. The many-armed diversity of cephalopods is the result of the evolutionary rubber hitting the road right after lineages split into new species, and precious little of their evolution has been the slow accumulation of gradual change. They aren’t alone. Sudden accelerations spring from the crooks of branches in evolutionary trees, across many scales of life — seemingly wherever there’s a branching system of inherited modifications — in a dynamic not examined in traditional evolutionary models. That’s the perspective emerging from a new mathematical framework published in Proceedings of the Royal Society B that describes the pace of evolutionary change. The new model, part of a roughly 50-year-long reimagining of evolution’s tempo, is rooted in the concept of punctuated equilibrium, which was introduced by the paleontologists Niles Eldredge and Stephen Jay Gould in 1972. “Species would just sit still in the fossil record for millions of years, and then all of a sudden — bang! — they would turn into something else,” explained Mark Pagel, an evolutionary biologist at the University of Reading in the United Kingdom. Punctuated equilibrium was initially a controversial proposal. The theory diverged from the dominant, century-long view that evolution adhered to a slow, steady pace of Darwinian gradualism, in which species incrementally and almost imperceptibly developed into new ones. It opened the confounding possibility that there was a discontinuity between the selection processes behind the microevolutionary changes that occur within a population and those driving the long-term, broad-scale changes that take place higher than the species level, known as macroevolution. In the decades since, researchers have continued to debate these views as they’ve gathered more data: Paleontologists hav...